Randomization A randomization list was prepared by a biostatistician who was not involved in data collection or clinical care of the infants. Lists of randomization assignments, which were kept in a sealed envelope in a locked drawer, were prepared for each of the five sites at which participants were recruited. Randomization assignments were made in blocks sizes of two and four. Once a study participant arrived home from the neonatal intensive care unit, a research assistant uncovered the next assignment on the randomization list, which was kept in a sealed envelope in a locked drawer.